st: Average for panel data

Dear statalisters,
I am working with an unbalanced panel Data. I?m trying to generate an average
measure with a database similar to this:
Id Year V1 V2 V3=(v2/v1)
1 1990 100 25 25
1 1991 500 60 12
1 1992 0 0 .
1 1993 . . .
1 1994 600 300 50
The average that I need should be the cumulative sum of the variable V3 divided
between the number of years with values different from 0 or missing. For example:
year1990: (25/1)
year1991: ((25+12) /2)
year 1992: ((25+12)/2)
year 1993: ((25+12)/2)
year 1994: ((25+12+50) /3)
Only considering those years with information different from 0 or missing value.
Thanks in advance for your help,
Sergio
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